Field of the Invention
The present invention relates to carbonaceous materials. In particular, the invention relates to mesoporous carbon black and processes for making mesoporous carbon black.
Discussion of Background Information
Mesoporous carbon black is carbon black having an average pore size that is greater than 2 nm, but less than the average primary particle size (Dp) of the carbon black. Mesoporous carbon black typically has a higher surface area than microporous carbon black, i.e., carbon black having an average pore size less than 2 nm.
Porosity in carbon black can be achieved via several methods. One common way of increasing the porosity of furnace blacks is to increase the residence time in the carbon black reactor, allowing the tail gas additional time to attack and etch the carbon surface. Another method is to add alkali earth metal ions to the carbon black feedstock, as these ions are known to catalyze the etching of the carbon black via the tail gas. Both of these techniques involve etching the carbon black “in-situ,” i.e., in the furnace reactor during production to create carbon blacks with internal porosity. The main limitations of extending the residence time in the carbon black reactor are the physical length of the reactor and the temperature; carbon black reactors are generally not long enough to obtain very high levels of porosity. In addition, at lower reactor temperatures (which may be used to make larger primary particles) the etching rate is not sufficiently fast. A limitation of adding alkali earth metal ions is that it will leave behind impurities on the carbon black, which could be undesired in some applications. Another major limitation of both these in-situ etching techniques is that, to date, they favor formation of microporous carbon black rather than mesoporous carbon black. To date, there is no known way of using traditional carbon black furnace reactors to generate highly mesoporous carbon black.
Some degree of mesoporosity exists in certain competitive carbon blacks made via a gasification process, where carbon black is a byproduct of the reaction (for example, Ketjen EC600, Ketjen EC300, or Printex XE-2). These products have significant mesoporosity but because of other factors related to the gasification process, the base morphology (i.e., primary particle diameter and aggregate diameter) that can be made via this process appears to be rather limited. These products typically have primary particle sizes Dp that are less than 30 nm. In addition, the structure of carbon black particles formed in the gasification process is generally high, meaning that the fractal carbon black aggregates contain large numbers of primary particles.
Thus, the need exists for processes for making mesoporous carbon black that also provide the ability to carefully control morphology (i.e. both primary particle and aggregate size). In addition, the need exists for mesoporous carbon black having pore and morphology characteristics different from carbon black that is formed by conventional gasification processes.